1. Field of the Invention
The present invention relates generally to arcuate enclosed electrolytic bubble gravity sensors, and more particularly, to means for mounting such sensors to reduce errors arising from ambient temperature variations.
2. Description of the Prior Art
Arcuate enclosed electrolytic bubble gravity sensors have conventionally been mounted with metallic clips similar to miniature fuse holders, or by cementing the sensor in a grooved metallic fixture. Referring to FIG. 1, another mounting technique entails positioning a post 10 with respect to an arcuate bubble gravity sensor 11 such that a longitudinal axis 12 of the post 10 is parallel to the plane of an arcuate axis 13 of the sensor 11. One end of the post 10, having an end face 14 disposed in a plane orthogonal to the longitudinal axis 12, is bonded with an epoxy cement 15 to the central region of a concave arcuate surface 16 of the sensor 11. The other end of the post 10 is bonded with the epoxy cement 15 to a base plate 17 such that the longitudinal axis 12 of the post 10 is orthogonal to the plane of the base plate 17.
The above mounting mechanisms have proven satisfactory for applications such as gyro leveling wherein gravity sensing errors entailing a few minutes of arc are relatively innocuous, in view of the myriad of other acceleration, mass shift, and friction force errors.
However, when the mounting apparatus depicted in FIG. 1 is utilized in inclinometers or in systems for measuring the tilt or angle of deviation of substantially stationary structures, the sensor exhibits, with changes in ambient temperature, unacceptable deviations. Referring to FIG. 2, with continuing reference to FIG. 1, the problem resides in the difficulty of positioning the end face 14 of the post 10 against the concave arcuate surface 16 of the sensor 11 such that the epoxy cement 15 is uniformly distributed therebetween. In practice, the cement joint has been found to vary in thickness over its length. Since the epoxy cement 15 generally possesses a relatively high temperature coefficient of expansion (TCE), on the order of 20-30 micro in./in./.degree.F., the additional expansion on one end of the cement joint produces angular displacement of the sensor 11 from the horizontal, resulting in erroneous readings. For example, if a width W of the post 10 is 0.200 inches, and the difference between a cement width A and a cement width B is 0.030 inches, which has been found to be typical, then for a 100.degree. F. change in temperature (.DELTA.T), the change in angle (.DELTA..theta.) is: EQU .DELTA..theta.=Tan.sup.-1 (B-A)(TCE).sub.W (.DELTA.T)=0.0215 degrees=1.29 arc min.
Further, in response to temperature variations, the base plate 17 has been found to warp transversely, as exaggerated in FIG. 3, imparting, through the post 10, additional undesirable angular displacement of the sensor 11 from the horizontal.
Thus, there is a need for an apparatus for mounting an arcuate enclosed electrolytic bubble gravity sensor, which reduces errors arising from ambient temperature variations.
In the present inventors' copending application Ser. No. 527,908, entitled Tilt Sensor Null Adjusting Apparatus, also assigned to the assignee of the present invention, a tool for precisely leveling a tilt sensor is described. While capable of adjustment to a fraction of an arc minute, it is apparent this inherent precision may not be realized with the prior art sensor mounting.